Standard conductivity measurement is one of the most common parameters in boiler feedwater and steam condensate monitoring. It is fast, inexpensive, and gives a useful general indication of total dissolved solids. But in power generation and co-generation applications, standard conductivity has a significant limitation: it cannot reliably distinguish between contaminants that are genuinely corrosive and background conductivity from carbon dioxide, which is largely harmless to metal surfaces.
This is the problem that cation conductivity was developed to solve — and understanding the difference between the two measurements is essential for anyone operating a boiler system where early detection of contamination is a protection requirement.
What standard conductivity measures
Standard conductivity measures the total ion concentration in the water — every dissolved species that carries an electrical charge contributes to the reading. In a steam cycle, this includes sodium, chloride, sulphate, bicarbonate, carbonate, ammonium ions from the amine-based treatment programme, and dissolved carbon dioxide that has formed carbonic acid.
The challenge is that carbon dioxide is almost always present in steam condensate systems. CO₂ enters the steam cycle from the deaeration of bicarbonate alkalinity in the boiler water, and it travels with the steam into the condensate return system. When steam condenses, the dissolved CO₂ forms carbonic acid, which dissociates to produce hydrogen and bicarbonate ions — both of which contribute to conductivity.
CO₂-derived conductivity is not trivially small. In many condensate systems, it accounts for 30 to 60% of the total conductivity reading. This means a standard conductivity sensor may show a reading of 0.3 µS/cm even when the water contains no chloride, sulphate, or sodium — purely from the CO₂ contribution. If the target for standard conductivity is, say, 0.1 µS/cm, the system will appear to be in violation even with perfectly clean water.
This masks the signal. If a contamination event introduces chloride into the condensate — through a condenser leak, for example — the chloride contribution to conductivity may be small relative to the existing CO₂ background, making the event difficult to detect against the noise.
What cation conductivity measures
Cation conductivity — also called acid conductivity or after-cation conductivity (ACC) — removes the ambiguity by passing the water sample through a cation exchange resin before measurement. The cation exchange resin replaces all positive ions in the sample (sodium, ammonium, calcium, magnesium) with hydrogen ions. This converts all the dissolved salts into their corresponding acids: sodium chloride becomes hydrochloric acid, sodium sulphate becomes sulphuric acid, and ammonium bicarbonate becomes ammonium-free carbonic acid.
Importantly, the carbonic acid produced from bicarbonate and CO₂ is a weak acid with low dissociation — it contributes only minimally to conductivity in the acid form. The ammonium ions added to the system as part of the amine treatment programme are also removed by the cation exchange, eliminating their contribution to the reading.
What remains in the sample after cation exchange is the conductivity contribution from the mineral acid anions — chloride, sulphate, and similar ions. These are the species that are genuinely corrosive to boiler internals, turbine blades, and steam pipework. The cation conductivity reading is therefore a direct indicator of the concentration of corrosive anions, without the interference from CO₂ or treatment chemicals.
Why this matters for condenser leak detection
A condenser leak introduces cooling water into the steam condensate. Cooling water typically contains significant concentrations of chloride and sulphate from the dissolved minerals in the makeup water supply. When this contaminated condensate returns to the boiler, the chloride and sulphate it carries concentrates in the boiler water as the steam evaporates — eventually reaching concentrations that cause pitting corrosion and stress corrosion cracking of boiler components.
Standard conductivity is a poor detector of small condenser leaks because the CO₂ background masks the incremental conductivity increase from the incoming chloride. A cation conductivity sensor, with the CO₂ interference removed, responds directly and immediately to the chloride contribution. A small condenser leak that adds 0.01 µS/cm of chloride-related conductivity to a standard reading of 0.3 µS/cm is effectively invisible. The same contamination on a cation conductivity reading of 0.02 µS/cm doubles the reading — immediately apparent.
Most boiler suppliers and boiler insurance standards specify after-cation conductivity as the monitoring parameter for condenser leak detection precisely because of this sensitivity advantage. A commonly cited limit is 0.15 µS/cm for cation conductivity in high-pressure boiler feedwater. Many boiler manufacturers include cation conductivity exceedance as a start-up interlock — the boiler will not be permitted to operate if cation conductivity exceeds the limit, because the risk of corrosion damage at elevated chloride concentrations is too high.
Degassed cation conductivity: the further refinement
Even after cation exchange, some residual CO₂ contribution remains in the cation conductivity reading. For very high-pressure, high-sensitivity applications — where the target cation conductivity is below 0.1 µS/cm — this residual can be significant.
Degassed cation conductivity (DCC) removes this residual by passing the post-cation-exchange sample through a degassing stage — typically a re-boiler or membrane degasser — before conductivity measurement. This removes the dissolved CO₂ entirely, leaving only the mineral acid contribution. DCC is the most sensitive form of conductivity measurement available for steam cycle monitoring and is specified in ASTM D4519 for condensate monitoring in power generation applications.
The Lecol SWAS (Steam and Water Analysis System) degassed cation conductivity units used in Autoflo’s boiler monitoring solutions comply with ASTM D4519, providing degassed cation conductivity measurement with integrated heat exchange and degassing stages in a compact, maintenance-friendly configuration.
When to use each measurement type
Standard conductivity is appropriate for general water quality screening, blowdown control, and applications where background CO₂ is not a concern — water treatment systems, cooling water circuits, and reverse osmosis permeate monitoring.
Cation conductivity (ACC) is the correct choice for boiler feedwater and condensate monitoring where condenser leak detection is required, where the amine treatment programme contributes to standard conductivity, or where standard conductivity limits are being violated by CO₂ despite good water chemistry.
Degassed cation conductivity (DCC) is specified for high-pressure boilers, power generation steam cycles, and any application where cation conductivity targets below 0.1 µS/cm are required and residual CO₂ interference in ACC must be eliminated.
Operating a boiler monitoring programme on standard conductivity alone, without understanding the CO₂ interference, is the equivalent of a smoke alarm that cannot distinguish between cigarette smoke and a house fire. The reading exists, but its diagnostic value is limited precisely when the information it needs to provide is most critical.
Autoflo Technology supplies boiler water quality monitoring systems including Lecol SWAS sample conditioning equipment with cation exchange and degassing capability. Contact us at info@autoflotechnology.com for more information.